A spacecraft may have several lithium-ion batteries, each of which having approximately thirty cells. The cells are sensitive to over-charging and over-discharging, thereby requiring complex protection circuits.
A common approach for charging the batteries and balancing charge of each cell includes the use of a power supply for each cell, totaling approximately 120 power supplies. As known in the art, it is desirable to minimize the number spacecraft system components and weight of the spacecraft. The above stated approach is complex and costly, especially for space applications, which are sensitive to weight and packaging constraints.
It is also desirable in space applications for systems to be reliable. Increasing number of battery system components adversely affects reliability of a battery system and ultimately a spacecraft itself.
Another battery charging method uses a primary/redundant charger and a primary/redundant discharger for the cells that are in series. A voltage clamp circuit is used to protect the cells from overcharging, and a disconnect switch and bypass switch protect the cells from over-discharging. This approach is also complex and costly. During charge of the cells valuable bus power is wasted when the voltage clamp is protecting the cells from overcharging. Also, the clamp circuit typically requires additional heat sinking to dissipate heat. There is a dependency on electro-mechanical devices to protect the cells from over discharging, and such devices have limited life cycles.
A dissipative cell balancing method has also been introduced, in which lithium-ion battery cells may be balanced on a spacecraft in a cost-effective manner. The method includes a resistor applied across each battery cell. Voltage across the cells is monitored by either a system operator or by computer. A resistor is disconnected when a desired voltage is reached across a cell of interest. Although, effective the method requires monitoring intelligence and some form of decision generation. Although this method may be cost effective it requires complex control techniques which tax limit control system intelligence. T he method uses complex computer software and significantly reduces onboard processor resources.
Additionally, a distributed converter method has been introduced including a balancing switch and a transformer/rectifier circuit associated with each cell of a battery. A multiplexer measures voltage of each cell. A controller operating the balancing switches such that each switch is in an “ON” state, or charging state, when voltage measured across a cell is below a first predetermined voltage level and in an “OFF” state when the cell voltage is above a second predetermined state. Although, this technique is cost effective it is also complex due to a need for control elements, thus reducing processor resources.
Prior art battery balancing techniques that preferentially charge a cell having a lowest state of charge are also inefficient. When a single cell is unable to charge, remaining cells are effected and are potentially not charged. A balancing system, using the preferentially charging technique, continuously attempts to charge the cell that is unable to charge, thus preventing other cells that have a state of charge higher than the unchargeable cell and lower than a desired state of charge from charging and the battery becomes inoperable.
Also, during discharge of a battery when a cell voltage drops below a certain value cell voltage reversal may occur. When cell voltage reversal occurs, cell potential inverts and the cell is shorted, also rendering the battery inoperable.
It is therefore desirable to provide a battery balancing technique that minimizes the number of system components, weight of a spacecraft, system complexity both in component quantity and control logic, and is cost effective and reliable.